# No.521 Cheeses and a Mousetrap(チーズとネズミ捕り)
n, k = [int(s) for s in input().split()]

if k == 0 or k > n:
    print(0)
elif n % 2 == 1 and n // 2 + 1 == k:
    print(n - 1)
else:
    print(n - 2)